1. Field of the Invention
The present invention relates to an antenna switching circuit, more particularly to an antenna switching circuit for enabling a transmitting circuit and receiving circuit to share one antenna and for switching the antenna to the transmitting circuit or receiving circuit at the time of transmission or reception and a communication apparatus configured by using the same.
2. Description of the Related Art
In a receiving terminal unit in a mobile communication system, for example, a terminal unit of a car phone or cellular phone system, a transmitting circuit and receiving circuit share one antenna. In this case, usually, an antenna switching circuit is provided between the antenna and the transmitting circuit and receiving circuit. The antenna switching circuit switches the antenna to the transmitting circuit side at the time of transmission and switches the antenna to the receiving circuit side at the time of reception.
In a terminal unit configured in this way, at the time of transmission, a high frequency transmission signal (RF signal) generated by the transmitting circuit is propagated to the antenna through the antenna switching circuit and is emitted into the air by the antenna as a radio signal. At the time of reception, the high frequency signal received by the antenna is supplied to the receiving circuit through the antenna switching circuit.
By providing the antenna switching circuit, the transmitting and receiving circuits can share one antenna and the, communication terminal unit can be made more compact and lighter in weight.
In a communication terminal unit having the above configuration, however, measures are required to prevent high frequency signal leakage caused by the antenna switching circuit. This is because, to prevent radio interference etc., it is required that the amount of emission of a high frequency signal other than of the transmission signal band from a communication unit be kept below a constant reference value. Therefore, in a communication unit, it is necessary to suppress the generation of a high-order harmonic signal in the circuit parts handling a high frequency signal, for example, in not only a high frequency amplification circuit and local oscillation circuit, but also the antenna switching circuit. In general, to suppress the high-order harmonic signal from the antenna switching circuit, measures may be taken such as improving the linearity of a transmission characteristic of the passband of the antenna switching circuit itself or providing a filter for removing the high-order harmonics at the antenna terminal side.
However, it is technically difficult to pass the high power RF signal generated by the transmitting circuit through the antenna switching circuit with low distortion due to the restrictions of the power supply voltage. Moreover, the signal is transmitted and received by different frequency bands in many cases. Since it is necessary for one antenna to handle high frequency signals of different frequency bands, the desired level of effect often cannot be obtained even if using a filter for removing the high-order harmonics at the antenna side.
Moreover, the high-order harmonic signal output to an antenna terminal of the antenna switching circuit, that is, signal output terminal, fluctuates due to the high frequency band impedance of the circuit connected to an input terminal. Usually, in a high-order harmonic measurement and evaluation path or the settings of a communication unit, the impedance at the high-order harmonic band of the circuit connected to the signal input terminal often is off from 50xcexa9. Therefore, the level of the high-order harmonics at the antenna terminal side fluctuates due to the snaking of the signal transmission line or the impedance of the circuits connected thereto. Depending on the evaluation conditions, there is a possibility of the set specifications not being met. It is sometimes not possible to effectively prevent leakage of a high-order harmonic signal from the antenna based on the evaluation results.
The present invention has been made to solve the above problems and has as its object to provide an antenna switching circuit, in a communication system where the transmitting and receiving circuits share an antenna, providing a filter for reflecting back high-order harmonic components between an antenna switch and a transmitting circuit to enable a reduction of the high-order harmonics generated from the antenna switch and a communication apparatus using the same.
According to the present invention, there is provided a communication system comprising: an antenna; a reception circuit receiving a signal received at the antenna; a transmission circuit sending a transmission signal to be transmitted to the antenna; a switch circuit provided between the antenna and the transmission circuit; and a filter circuit provided between the switch circuit and the transmission circuit.
The filter circuit reflects a signal component having a predetermined high frequency band in the transmission signal.
The filter circuit reflects a signal component having a predetermined high frequency band in the transmission signal to the switch circuit.
A first signal transfer line having a predetermined impedance is provided between the switch circuit and the filter circuit.
A second signal transfer line is provided between the transmission circuit and the filter circuit.
According to the present invention, there is also provided a communication system comprising: an antenna; a reception circuit receiving a signal received at the antenna; a transmission circuit sending a transmission signal to be transmitted to the antenna; a switch circuit provided between the antenna and the transmission circuit; and a filter circuit provided among the switch circuit, the transmission circuit and the reception circuit, said filter circuit passing a fundamental wave component in the transmission signal and reflecting a signal component having a predetermined high frequency band in the transmission signal.
The antenna switching circuit of the present invention is an antenna switching circuit for enabling a transmitting circuit and a receiving circuit to share single antenna and for switching the antenna to the transmitting circuit at the time of transmission or the receiving circuit at the time of transmission or reception, comprising an antenna switch connecting the antenna to the transmitting circuit at the time of transmission and connecting the antenna to the receiving circuit at the time of reception and a filter circuit which is connected to at least a signal transmission line between the antenna switch and the transmitting circuit and reflects back a signal of a predetermined high frequency band propagated through the signal transmission line.
Further, the communication system of the present invention is a communication system for transmission and reception sharing single antenna, comprising a transmitting circuit for transmitting a high frequency signal by using the antenna; a receiving circuit for receiving a radio signal by the antenna; an antenna switch for connecting the antenna to the transmitting circuit at the time of transmission and the antenna to the receiving circuit at the time of reception; and a filter circuit which is connected to at least a signal transmission line between the antenna switch and the transmission circuit and reflects back a signal of a predetermined high frequency band propagated through the signal transmission line.
In the present invention, preferably, a high-order harmonic component generated in the antenna switch corresponding to a transmission signal output by the transmitting circuit is propagated toward both the antenna and the transmitting circuit, and the high-order harmonic component propagated toward the transmitting circuit is reflected by the filter circuit and propagated toward the antenna through the antenna switch.
In the present invention, preferably, an amplitude of the high-order harmonic component reflected by the filter circuit is controlled by a reflection characteristic of the filter circuit or a phase of the high-order harmonic component reflected by the filter circuit is controlled by a length of the signal transmission line between the filter circuit and the antenna switch.
In the present invention, preferably, the reflection characteristic of the filter circuit and a length of the signal transmission line between the antenna and the antenna switch are controlled so that the high-order harmonic component propagated from the antenna switch in the signal transmission line between the antenna switch and the antenna and the high-order harmonic component reflected by the filter circuit become substantially the same in amplitude and inverse in phase.
According to the present invention, the wireless communication terminal unit with transmitting and receiving circuits sharing one antenna is provided with a filter circuit allowing the passage of a fundamental wave propagated over a signal transmission line between an antenna switch and the transmitting circuit, exhibiting a high impedance with respect to a high-order harmonic component of the fundamental wave generated by the antenna switch, and reflecting back the high-order harmonic signal. A high-order harmonic component is generated by the antenna switch in accordance with the transmission signal from the transmitting circuit at the time of transmission due to the linear distortion of the antenna switch etc. and is propagated toward the antenna and transmitting circuit. The high-order harmonic component propagated toward the transmitting circuit by the filter circuit provided on the signal transmission line between the antenna switch and the transmitting circuit is reflected back, propagated again toward the antenna switch, and further propagated toward the antenna through the antenna switch.
By controlling the reflection characteristic of the filter circuit, for example, the reflectance, and further by controlling the signal transmission line length between the filter circuit and the antenna switch, the high-order harmonic component generated by the antenna switch and the high-order harmonic component reflected by the filter circuit are controlled to become substantially the same in amplitude and inverse in phase on the signal transmission line between the antenna and the antenna switch, so the high-order harmonic components on the signal transmission line are substantially removed and the emission of a high-order harmonic signal from the antenna can be suppressed.